The invention relates to a detection wafer with a non-linear circuit for an electromagnetic detection system wherein two different transmision frequencies are used, comprising a resonant circuit connected to a non-linear element forming from the transmission frequencies a third frequency to be detected. A similar detection wafer is known from German patent application No. 2,750,863. This known detection wafer is adapted to form, starting from two signals having frequencies f.sub.1 and f.sub.2, respectively, and transmitted in an interrogation zone by a transmitter associated with the detection system, a signal having a third frequency f.sub.3. This third signal can be detected and then indicates the presence of a detection wafer in the interrogation zone. The known detection wafer is provided for that purpose with a resonant circuit comprising a non-linear element, such as a diode. This diode forms from the received signals having frequencies f.sub.1 and f.sub.2 a number of distortion components. The frequencies f.sub.1 and f.sub.2 are chosen in such a manner that these frequencies both lie within the resonant frequency bandwidth of the resonant circuit.
The frequency f.sub.1 may e.g. coincide with the peak of the resonance characteristic of the resonant circuit and the frequency f.sub.2 may be slightly higher than the frequency f.sub.1 and be chosen in such a manner that at the level of f.sub.2 the resonant characteristic has decreased to a relatively slight degree.
If furthermore the signal having the frequency f.sub.1 has the largest amplitude of the distortion components, the signal having a frequency f.sub.3 =2f.sub.1 -f.sub.2 is the strongest representative in the current through the coil of the resonant circuit, for this frequency likewise lies in a range where the resonance characteristic of the resonant circuit has decreased only slightly. The presence of a signal having the frequency f.sub.3, which indicates the presence of a detection wafer in the interrogation zone, can now be simply detected with a narrow band filter with central frequency f.sub.3.
Although the known detection wafer is quite satisfactory for many applications there is nevertheless sometimes a need for a stronger signal having the frequency f.sub.3, so that there may be a greater chance of detection. Further, it is desirable that both the known detection wafer and the detection wafers providing a greater chance of detection can be employed together with the same detection system used already for the known detection wafers.